What does primary and secondary chord mean in music?

In music, primary chords refer to the three most important chords in a key, typically the tonic (I), dominant (V), and subdominant (IV) chords, which form the foundation of harmony. Secondary chords, on the other hand, are chords that function as temporary substitutes or extensions, often derived from other scales or keys, and can add complexity and color to a piece. They are often used to create tension or lead back to the primary chords.